The volume seemed to be turning up: Gillian Welch and Dave Rawlings were moving away from acoustic austerity, growing comfortable rocking out. Their last album, "Soul Journey," saw these Nashville alt-folkies jolting their sound with electricity and a backbeat. Rawlings has stepped out as a producer — indie rockers Bright Eyes, the shambling Old Crow Medicine Show — and the duo guested on the Decemberists' breakout, "The King Is Dead." Could this mid-career acoustic duo — in its first release in eight years — be ready for a similar lunge into the mainstream?

Well, probably not. With their new album, "The Harrow & the Harvest," the two have done the opposite of what folkie Bob Dylan did when he plugged in at Newport in 1965: They've made room for a quiet, contemplative record that includes desperation, heartache and resolve.

And this couple has also brought their musical union — of two voices, two interlocked acoustic guitars — to an even finer pitch: On songs such as "Dark Turn of Mind" and "The Way the Whole Thing Ends," they make the Stanley Brothers sound estranged.

It was that tradition of Appalachian brother teams to which the two — Rawlings describes himself as part of a two-piece band called Gillian Welch — were hoping to return.

"We were very committed to the duet this time," says the lank, fair-skinned Welch, relaxing next to her 1956 Gibson guitar outside the Echo Park home of Conor Oberst. "We had a lot of pent-up fire for it; we'd been away from it for a long time. So that's the first thing we knew about this record — that there was not gonna be anyone else on it."

"Harrow" is not only their first release in eight years, it's also their first album in the acoustic style with which they've been associated since 2001's "Time (The Revelator)." Their tour, which comes to the Music Box in Hollywood on Thursday, will be similarly spare, with just the two onstage, strumming and singing in an almost religious reverie.

Despite the years of creative drought — the two kept writing songs, none of which struck them as stylistically balanced or emotionally true — they've grown even closer as musicians, and they're now a hip "influence" on the new acoustic scene. The All Music Guide calls the new album "stunning in its intimacy, its lack of studio artifice, its warmth and its timeless, if hard won, songcraft."

Regional as universal

From the beginning of her career, around her T Bone Burnett-produced 1996 debut, "Revival, " Welch has been dogged by accusations based on her origins. On the album cover she looks like something out of a Walker Evans photograph, but she is a New York City native, former Westsider and graduate of Santa Monica's elite Crossroads School. Her adoptive parents were not Dust Bowl refugees but a former comedy team who wrote music for "The Carol Burnett Show."

Even before being asked about it directly, Welch is a bit defensive on the matter. "Man, I grew up in my room," she says. "I could have been anywhere. I grew up in my room, with my record collection."

Her memories of Los Angeles are vague — they mostly involve, she says, running around on the fire roads above her parents' hillside home and doing ceramics in high school. But one aspect of her childhood was crucial: What she calls her "hippie grade school" — Westland on Mulholland Drive — where her class sang Carter Family and Woody Guthrie songs nearly every day.

In one key way, in fact, she had a pre-modern upbringing: While she grew up singing classic folk and country music, she never heard recordings of them. She knew the songs only through an oral tradition. Years later, she attended UC Santa Cruz, and was at first a bit lost, personally and musically — she sang briefly in front of a psychedelic surf band — but was exposed to old recordings by a bluegrass deejay housemate.

"And I have this complete, strange epiphany," she recalls. "I'm hearing these sounds I've never heard before, but they're the songs I've known since I was a kid." The experience turned her head around. "And then presto, we're done. And then I'm just like every other person who just needed a record player, these records and a door that locked."

Rawlings, meanwhile, was growing up a Neil Young freak in a Rhode Island mill town. The offspring of a long line of engineers, he's earnest and solicitous, with the temperament of both a poet and a mathematician.

"I can talk about people I like," he says when asked about his distinctive single-note crosspicking. "But my preference for particular notes is something born into me — that did not come from listening to anyone."

Beyond tradition

"The Harrow & the Harvest" is a purist's return to the prewar music that brought these two together as students at the Berklee College of Music in the early '90s. "There were 10 of us, in the entire college, who knew who the Stanley Brothers were," Welch says of those dark days before the "O Brother, Where Art Thou?" soundtrack.

"This is something that Dave and I kind of inherited from that tradition," she says. "As a duet, you do everything in your power to change the texture of what you're doing — you're constantly fighting stasis. You only have four things" — two guitars, two voices. "This is part of why we gravitated to the dissonance in our music — why Dave will often sing dissonant intervals."

In a lot of ways, this duo — two nocturnal musicians who walk around their adopted hometown of Nashville after midnight, dreaming of the ghost of the Everly Brothers, Elvis Presley and Hank Williams — has come full circle.

"When we started down this road, around the time of 'Revival,'" Rawlings says, "we felt we were very much alone in the acoustic wilderness. There wasn't a lot of stuff that we felt connected to us. Celine Dion was cutting the 'Titanic' vocal in the studio while we were making our little record. We just felt like freaks.

"It's surprising to look up years later and see all these people working in this genre — Fleet Foxes, Bon Iver, Mumford and Sons. It's surprising to see us and go, 'Whoa, we're veterans.'"